Turquoise is an opaque, blue-to-green mineral that is a hydrousphosphate of copper and aluminium, with the chemical formulaCuAl6(PO4)4(OH)8·4H2O. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue.
In many cultures of the Old and New Worlds, this gemstone has been esteemed for thousands of years as a holy stone, a bringer of good fortune or a talisman. The oldest evidence for this claim was found in ancient Egypt, where grave furnishings with turquoise inlay were discovered, dating from approximately 3000 BCE. In the ancient Persian Empire, the sky-blue gemstones were earlier worn round the neck or wrist as protection against unnatural death. If they changed color, the wearer was thought to have reason to fear the approach of doom. Meanwhile, it has been discovered that turquoise can change color. The change can be caused by light, or by a chemical reaction brought about by cosmetics, dust, or the acidity of the skin.
Turquoise is a stone and color that is strongly associated with the domes and interiors of large mosques in Iran, Central Asia, and Russia.[5][6]
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Maerz; Paul (1930). "Color Sample of Cyan: I". A Dictionary of Color. New York, NY: McGraw-Hill. page 73, plate 25, sample K12. — 'Turquoise blue' is shown lying very close to Turquoise, but very slightly more bluish.